1. Field of the Invention
The present invention relates to a DC coupled transmitter, and in particular to a DC coupled transmitter including a tunable source termination circuit that can be enabled/disabled with an internal supply voltage less than or equal to the sink termination voltage and without device stress.
2. Related Art
In deep sub-micron processes, the maximum I/O voltage is beginning to scale along with the technology. For example 3.3 V I/O supply and devices are commonly available in 90 nm, but in 65 nm and beyond, the migration to 2.5 V and even 1.8 V input/output (I/O) devices and supply is occurring rapidly. This scaling creates a problem for conventional implementation of source termination in a high performance DC coupled transmitter, such as a high-definition multimedia interface (HDMI™) transmitter, where the far-end termination voltage is 3.3 V.
FIG. 1 illustrates a simplified HDMI system including a transmitter PHY 101 and a receiver PHY 104 (wherein PHY refers to the physical layer). An exemplary transmitter PHY 101 could represent an audio/visual source (e.g. a set-top box, DVD player, PVR, etc). As shown in FIG. 1, transmitter PHY 101 includes a transmitter 102 having a differential pair of pins txp and txm. An exemplary receiver PHY 104 could represent an A/V monitor (e.g. a digital television (DTV) etc.). Receiver PHY 104 includes a receiver 106 and a far-end termination 105.
Notably, the HDMI standards (V1.3 and V1.4) specify that the far-end termination voltage should be 3.3 V±5%. Unfortunately, implementing the transmitter output stage with a lower supply (e.g. 2.5 V or 1.8 V) can lead to reliability problems, leakage paths (see leakage path 110 from receiver PHY to transmitter PHY in FIG. 1), and even an inability to enable/disable termination. Addressing these problems, while meeting all the necessary HDMI specifications with regards to eye diagram mask and source DC characteristics (e.g. single-ended output voltage swing and level) has proven to be difficult.
One conventional approach to resolve these problems while meeting design specifications is to use a differential fixed termination external to transmitter PHY 101. This non-integrated solution is extremely costly and therefore not commercially viable. On the other hand, a fixed termination internal to transmitter PHY 101 can easily violate HDMI specifications where source termination is prohibited at data rates below 1.65 Gbps.
Another conventional approach is to have a dedicated near-end 3.3 V supply for the transmitter output stage. This configuration allows for a relatively straightforward source termination, but does not eliminate the leakage paths in the case when the near-end 3.3 V transmitter voltage is removed and the far-end termination voltage is still engaged (which occurs in actual HDMI applications). Furthermore, requiring a dedicated 3.3 V transmitter supply voltage is contrary to the current migration to low voltage I/O devices.
Yet another conventional approach is not using any termination. However, reflections caused by large impedance mismatch between the near-end and the channel (or cable) will be an issue at high data rates and could violate current HDMI electrical specifications.
Therefore, a need arises for a commercially-viable source termination for an HDMI application that can be safely enabled/disabled with the near-end termination voltage (e.g. 2.5V) lower than the far-end termination voltage (e.g. 3.3V). Also, a need arises for a source termination that can provide a stress free condition where the junction voltage is less than the maximum I/O voltage, regardless of the state of the receiver PHY.